1. Field of the Invention
The present invention relates to a dielectric filter comprising a plurality of dielectric coaxial resonators arranged in parallel.
2. Prior Art
There are known dielectric filters of the type comprising a rectangularly parallelepipedic dielectric block provided with a plurality of through holes bored from the top all the way to the bottom thereof, the peripheral walls of the through holes being coated with an inner conductor to produce so many dielectric coaxial resonators, an outer conductor arranged on a substantial portion of the outer peripheral surface of the dielectric block and a short-circuiting conductor arranged on the surface of the top of the dielectric block and connecting the outer conductor on the outer peripheral surface and the inner conductors on the peripheral walls of the through holes to make it a short-circuiting side, the bottom of the dielectric block being left as an open-circut end. Such dielectric filters can suitably be used as high frequency band filters.
FIG. 1 of the accompanying drawings shows a conventional dielectric filter comprising a rectangularly parallelepipedic dielectric block A which has a short-circuiting end surface B and is provided with paired resonators C, wherein there is provided a region D free from the short-circuiting conductor on the top of the dielectric block A, or the short-circuiting end surface B, between the paired resonators C as interstage coupling means of the resonators (see Japanese Patent Kokai No. 3-293802).
Japanese Patent Kokai No. 8-8607 filed by the applicant of the present patent application discloses a further conventional dielectric filter for a high frequency band. In this conventional dielectric filter, as illustrated in FIG. 2, a dielectric block 1 is provided with a pair of through holes 2a and 2b arranged in parallel with each other, the peripheral walls of the through holes 2a and 2b are coated with respective inner conductors 3a and 3b to produce so many dielectric coaxial resonators. An outer conductor 4 is arranged on a predetermined area of the outer peripheral surface of the dielectric block 1, and a short-circuiting conductor 5 is arranged on the surface of the bottom of the dielectric block 1 for connecting the inner conductors 3a and 3b with the outer conductor 4 to make it a short-circuiting side. The top of the dielectric block 1 is left as an open-circuit end 6. Further, a slot 7 is formed by removing the outer conductor 4 in a straight and narrow zone on the outer peripheral surface located close to the short-circuiting conductor 5 and running in a direction perpendicular to the through holes 2a, 2b. An input/output terminals denoted by 8a and 8b are provided on the outer peripheral surface located close to the open-circuiting end surface 6 of the dielectric block 1.
With such an arrangement, the dielectric coaxial resonators can be coupled with each other and polarized easily and simultaneously.
Of known dielectric filters for high frequencies of the type under consideration, the one having a region, or slot, free from a short-circuit conductor on the short-circuiting side of the dielectric block for coupling each other as shown in FIG. 1 is required to make the slot show a large width and a surface area equal to about two thirds of the total surface area of the short-circuiting side in order to couple the resonators sufficiently at the cost of reducing the value of Q.
Additionally, the overall mechanical strength of the dielectric filter can be reduced if the coupling slot is formed by cutting the dielectric block at the short-circuiting side between the resonators.
On the other hand, the one an shown in FIG. 2 is typically covered with a shield casing after mounting the dielectric filter on the surface of a printed circuit board not shown and connecting the input/output terminals 8a and 8b formed on the lateral side of the dielectric block 1 opposite to the lateral side where the coupling slot 7 is formed directly to the printed circuit, said former lateral side being the under side of the dielectric block 1 with this mounting arrangement. Then, the inner wall surface of the shield casing has to be separated from the coupling slot 7 on the upper side of the dielectric block 1, or the lateral side where the coupling slot 7 is formed, by at least 3 mm because the degree of coupling can be altered if the inner wall surface of the shield casing comes closer to or into contact with the coupling slot 7 on the upper side of the dielectric block 1. In other words, this dielectric filter is not adapted to height reduction commonly required to dielectric filters of the type under consideration.
Additionally, when manufacturing dielectric filters for a high frequency band of the type under consideration, a number of devices (e.g., 20 to 30 devices) are pinched together by a set of jigs and silver paste is applied thereto to form a conductor layer on each of the devices for the purpose of mass production.
Then, a dielectric filter having a configuration as illustrated in FIG. 2 is accompanied by the problem that the stencil for producing a coupling slot thereon cannot be arranged accurately in position when the filter devices pinched by the jigs show positional displacements and/or dimensional deviations from the specified values, if slight.
In view of these and other problems, it is therefore an object of the present invention to provide a dielectric filter having a short-circuiting surface on which a coupling element can be precisely positioned without requiring any accurate patterning operation and capable of providing a sufficient degree of coupling without reducing the Q value and meeting the requirement of height reduction.